Driven by the increasing prevalence of digital content and multi-media applications, of late there has been a dramatic growth in the need for home networking. This has fuelled new development of home networking technology both wired and wireless. One such technology—the Multimedia over Coax Alliance (MOCA) V1.0 Specification (identified herein as [1]) describes the MAC and PHY layers for high-rate communications over the coaxial TV-cable plant that is present in most homes. MoCA makes use of a 256-tone OFDM based-PHY to provide for a data-rate of up to 310 Mbps at a range of up to 300 feet on a 50 MHz channel located in the 850˜1500 MHz band. This translates to an effective MAC-layer throughput of 125 Mbps.
Among other enhancements, MoCA V1.1 Draft Specification (identified herein as [2]) provides for several additions to the MAC-protocol in [1] that facilitate the realization of even higher MAC-throughputs. This is achieved primarily through the use of packet aggregation which allows for the overhead involved in a transmission to be shared among several packets and results in throughputs of up to 180 Mbps.
FIG. 1 depicts a typical MoCA network 120 comprising of MoCA nodes 111 through 115 connected over segments of coax through splitters 102, 103 and 104. The network connects to the external operator network through the Multi-tap 101, with the splitter 102 constituting the root-node. Within 120, nodes 111, 112, 113, 114 and 115 may communicate with each other. When the communications path between two nodes, for instance between 111 and 112 traverses two branches of the same splitter (in this example 103), the channel is referred to as ‘splitter-jumped’. Another example of ‘splitter jumping’ is at splitter 102 on the path between nodes 112 and 114.
While the physical MoCA network operates on a shared coax-medium, the logical network model is significantly different. Because of the effects of splitter-jumping and reflections at different terminations, the channel characteristics for a link between two nodes may be significantly different from the link between two other nodes. Channel characteristics between two nodes may also be different in the forward-path and the reverse-path. In order to maximize channel capacity, the MoCA MAC protocol defines operation of the MoCA network as a fully meshed collection of point to point links, each with its own channel characteristics and capacity. In order to effect communications, a node uses a set of optimized PHY parameters tailored to the channel characteristics between it and the destination node(s) that it wishes to communicate with. This optimal set of parameters is referred to as a ‘PHY-profile’ and is obtained based on a process known as ‘modulation profiling’ described in detail in [1].
Further, as a result of the channel being a physically shared medium, only one node may be transmitting at any given time. This is ensured by the fact that the MoCA network is a centrally coordinated system, with one of the nodes being assigned the responsibility of being the network-coordinator (NC), which in addition to transacting data on the network like existing-nodes (ENs) is also responsible for: transmitting beacons to advertise network presence and timing; coordinating the node-admission process whereby new-nodes (NNs) join the network; scheduling and coordinating the transmission of data among all nodes; and scheduling and coordinating periodic link-maintenance operations (LMOs) whereby all nodes update their PHY-profiles; among other functions, more fully described in [1] and [2].
FIG. 2 depicts the allowed frequency channels on which the MoCA-device may operate as per [1]. Literals 201 through 214, respectively define the centre frequency of different coax-channels on which a MoCA node may form a network as per [1]. All in all, there are 14-channels. These channels are enumerated as {A1, B1, C1, C2, C3, C4, D1, D2, D3, D4, D5, D6, D7, D8} respectively.
When a MoCA-node first powers up, it attempts to join an existing network by ‘scanning’ the frequency range 201 through 214 to detect beacons indicating the presence of a network on a given channel. Beacons are transmitted using a special PHY-profile referred to as ‘Diversity-Mode’ which is known a-priori to all nodes, hence allowing the beacon to be deciphered prior to performing modulation-profiling, which is performed as a part of the node-admission sequence. Diversity-Mode transmissions are additionally robust yielding the highest range of transmission on the MoCA network, albeit at a lower data rate, facilitating easy network detection by all nodes.
On detecting a beacon, the node synchronizes itself to the network-time, information of which is contained in the beacon. This is followed by the transmission of an admission-request and the execution of the rest of the node-admission process, as described in [1]. During node-admission, the NC schedules the creation of PHY-profiles and the exchange of transmit-power and capability-information between all the nodes in the network. Once a node is admitted on to the network, its classification changes from NN to EN and it starts to receive its schedule information through the MAP-frame (Media Access Plan) the pointer to which is initially conveyed through means of the beacon.
In the event that a node does not detect the presence of another network as part of its frequency scan, it assumes the role of the NC and starts to transmit beacons of its own for a short duration on each channel before switching to another channel. Thus several MoCA nodes scanning the network would eventually detect each other and form a network with one of them being the NC.
The standard [1] describes a frequency scan pattern and protocol which minimizes the chance of network-fragmentation—i.e. when some nodes form a network on one channel and others on another channel(s), rather than all on the same channel. Alternately, the standard [1] also allows for devices to be configurable such that their frequency-scan is limited to a subset of channels 201 through 214.
Additionally, the standard [1] defines the concept of ‘taboo-channels’ which are channels on which if transmissions occur, devices in existing network may suffer from impaired operation. The NC broadcasts its taboo-channel information in its beacon, informing other nodes not to form-a-network/transmit-on-the-taboo-channel. Likewise, NNs joining a network have their own taboo-channel information, which they inform the NC of via means of their admission request frame. The NC continually updates its taboo channel information with the union of the taboo-channel information of all nodes that attempt to join the network, thereby ensuring that a device that has received a beacon from the NC would not attempt to transmit on any of the channels that are taboo for any of the nodes in the network.
Within the network, data transmission between nodes is scheduled through means of a Media Access Plan (MAP)-cycle. Each MAP-cycle is described by means of a MAP-frame that is transmitted by the NC in the previous MAP-cycle. The MAP-frame is used to primarily convey information about the upcoming transmissions on the medium, link-state of the network, LMO operations and privacy encryption keys in effect for the duration specified in the MAP. In order to improve bandwidth efficiency on the network, the MAP-frame is transmitted using the NC's ‘MAP-Profile’ which is the greatest common denominator (GCD) of all the profiles between the NC and all ENs, hard-limited to 64 QAM and is expected to yield a PHY-data-rate greater than that of the Diversity-Mode profile. As the GCD-profile is derived from the PHY-profiles of the NC to the individual nodes (ENs) it can be received by each node in the network.
FIG. 3 represents the timing relations of the MAP-frame. As can be seen from the figure, each MAP-cycle is defined by a MAP-frame—311, 312 and 313, respectively occurring in the previous MAP-cycle 301, 302 and 303, respectively. The beacon 321 contains pointers to the Admission Control Frame (ACF) 322 as well as to the next-beacon 323 in order to facilitate node-admission for a NN. It also contains pointers to the next MAP-frame 312 which can be used by an NN which is completing node-admission and transitioning to an EN.
The MAP-frame represents the scheduling-unit for traffic on a network. The NC schedules reservation-request (RR) transmission-opportunities for different nodes by which they may make a request to transmit data in the next MAP-cycle. A single RR-frame may carry requests for multiple data units. Based on the RR-frames 331 and 332, the NC generates the MAP-frame 311 which defines the schedule of transmission in MAP-cycle 302. It may be noted that subject to bandwidth availability, the NC may accept or deny a node's reservation-requests. As can be seen from the figure, in addition to scheduling the transmissions requested by the RR-frames i.e. 341 and 342, the NC also schedules control-frames such as Beacon 321, RR-Z 333, ACF 322 and MAP-frame 312 in MAP-cycle 302.
In order to define minimum requirements for MoCA implementations, [1] prescribes time constraint TRM 351 which represents the minimum duration from the last RR of the current MAP-cycle to the MAP-frame defining the next MAP-cycle. This represents the amount of time the NC has available to it to compute the schedule for the next MAP-cycle. [1] also defines time-constraint TMR 352 which represents the minimum amount of time that ENs have to interpret the MAP-frame 312 and schedule their transmissions and receptions in the next MAP-cycle.